1. Field of the Invention
The present invention relates to sodar apparatus, methods and systems for use in detecting, characterising, recording and/or displaying the wake vortices shed from large aircraft on the approach to or departure flight path of an airport runway. It is more particularly concerned with sodar systems of the bi-static type where the acoustic receiver is down-range of the atmospheric echo source of interest. Thus, the echoes of interest are forward scattered, reflected and/or refracted having regard to the direction of the interrogating acoustic beam. This is to be contrasted with a mono-static system in which the transmitter and receiver tend to be co-located and the echoes of interest are those that are backward scattered, reflected and/or refracted.
2. Description of Related Art
Bi-static sodar systems have long been proposed and used for the sounding of the lower atmosphere where atmospheric discontinuities tend to be horizontal, slow moving and wide spread, except in storm conditions. This allows the use of a single transmitter and a single receiver located down-range from the transmitter by a known distance and the use of simple triangulation to determine the height of a source of ‘echoes’ from forward scattered, refracted and/or reflected transmitted signals. It is well known in such systems to track atmospheric anomalies over long periods of time by using a pulse transmitter and listening for echoes between pulses. Sufficiently strong echo signals allow Doppler components to be extracted that provide an indication of the velocity of movement of echo sources either vertically or down-range.
However, conventional bi-static sodar systems of the type indicated are unsuited to the characterization of wake vortices which are typically close to the ground and capable of rapid movement in three dimensions over their relatively short durations. The simple triangulation techniques of bi-static sodar are inadequate for such an application.
A serious and more general problem with the use of sodar techniques for atmospheric sounding arises from their inherently poor signal-to-noise ratio [s/n] due to (i) the limited power of acoustic transmitters (ii) the strong attenuation of acoustic waves in the atmosphere and (iii) the prevalence of acoustic noise. The latter problem is of particular importance in a noisy airport environment, especially when attempting to detect wake vortices of aircraft as they pass down (or up) the flight path.
The above problems have been addressed in our prior international applications PCT/AU01/00247, PCT/AU02/01129, PCT/AU04/00242 that disclosed sodar systems with exceptionally high s/n ratios. Our prior systems have variously employed long duration transmitted pulses encoded in a ‘pulse compression’ manner, over-sampling of received echoes for good resolution and processing gain, and the use of matched filter tailored to the pulse-compression code to provide low-power, long range sodars capable of extracting excellent Doppler signals from the received echoes. The pulses—generically called ‘chirps’—employed in our prior inventions preferably had durations in the order of tens of seconds. The pulse-compression technique employed was preferably a linear increase or decrease in phase (tone) over the duration of the pulse; for example, a steady increase in tone from 500 to 1500 Hz, or a steady decrease in tone from 1500 to 500 Hz. The methods disclosed involved ‘listening while sending’; that is, echoes are received and processed while transmission of the chirp is still under way. This technique not only allows very high system and processing gains that result in exceptionally good s/n (signal to noise ratio), but it also enables atmospheric discontinuities that occur close to the ground to be detected.
The present specification should also be read in conjunction with our international patent application PCT/AU2004/001075 entitled “Detection of Wake Vortices and the Like in the Lower Atmosphere” which taught the technique of sounding the atmosphere near an airport runway when a wake vortex is not present to generate a reference dataset, then sounding the atmosphere when it is suspected that an wake vortex might be present to generate an active dataset and differencing the two datasets to highlight a vortex, if present. Finally, this specification should be read in conjunction with our co-pending Australian patent application entitled “Staged Sodar Sounding” (to be published), which teaches sodar techniques wherein a set of long chirps is employed in a ‘send-then-listen’ mode in which the echoes generated by the pulses are extracted using matched filter methods. While the sodar systems disclosed in our prior applications were capable of detecting wake vortices and of monitoring wind conditions in the vicinity of airports with much greater sensitivity and precision than was previously possible, they still left something to be desired in tracking an intense wake vortex as it moves in three dimensions.
For brevity, the disclosures in our published applications are regarded as being incorporated herein, including the extensive discussion of the prior art contained in the specifications of those applications. In addition, some of the terminology that is used herein is explained or defined in those specifications.
In the following, the term ‘flight path’ will be used to designate the volume of the lower atmosphere near either end of an airstrip or runway through which aircraft pass on approach to or take-off from an airport. It is here where persisting wake vortices are caused by large aircraft and can be dangerous for other smaller aircraft using the flight path even minutes later.